## Wednesday, April 29, 2020

### How do it know? What determines current flow?

The crew breaks for lunch. George pours himself a cup of coffee. He says, "That coffee is hot!". Lunch break the next day George pours himself a cup of tea. He says, "Wow that tea is so cold it gave me an ice cream headache.". George asked the crew,"How does it know?". Alvin says "What know what George?". George says, "When My wife sends coffee it keeps it hot. When she sends tea it keeps it cold. How do it know to keep it hot or cold?".
Of course it does not know but it is an interesting question. We have a container inside another container and an insulator between. In order to pass through the insulation the energy would have to overcome the resistance the insulator offers to energy movement. Thermal conductivity of the insulator will determine the rate the temperature equalizes.
What would happen if we drilled a hole through the insulator and inserted a metal rod? The thermal conductivity of metal is very high so the heat would travel the rod and the inter camber would loose heat much quicker. If we knew the temperature and volume of the liquid we could determine the number of calories needed to make the inside and outside reach equilibrium. If one 1/4" rod would bring equilibrium in an hour how long would it take with 2 1/4" rods? What if we used 2 rods that were 1/4" and 1/2"? The larger rod would convey the energy faster than the smaller. We could total the cross sectional area of the rods and determine the rate. A rod that is twice as large would have 4 times the cross sectional area.
Now consider a circuit with a battery supplying an open circuit. It would be like the first case. With no circuit the current would not flow. Add a large value resistor and it will allow a small flow. Add another resistor and you have a higher current flow and so it goes.
How do it know? The circuit must have a path and it will produce as much flow as it can. Adding more paths produces more flow.
It knows one thing push as hard as it can and find any path available. After that it self regulates.